1. Field of the Invention
The present invention relates to an oscillating apparatus such as an ultrasonic oscillator. In particular, the invention relates to an ultrasonic oscillator applicable to an ultrasonic bonding machine.
2. Description of the Prior Art
An ultrasonic oscillator is well known as disclosed in Japanese Patent Application Publication No. 2001-340809. The ultrasonic oscillator utilizes a direct digital synthesizer, DSS, for generating a waveform signal. A digital phase lock loop (PLL) circuit is connected to the direct digital synthesizer. This structure allows the driving signal output from the direct digital synthesizer to follow the change in the resonant frequency of the transducer even if the resonant frequency varies. The oscillation of the transducer can in this manner be maintained. A larger gain of the phase lock loop circuit enables a quick response of the driving signal to the change in the resonant frequency.
A so-called ultrasonic bonding machine is well known in the technical field of the circuit chip mounting. The phase lock loop circuit is connected to the transducer of a so-called ultrasonic head in the ultrasonic bonding machine. If the gain of the phase lock loop circuit is excessively larger, the ultrasonic head cannot follow the change in the frequency, so that the action of the ultrasonic head gets disordered. On the other hand, if the gain of the phase lock loop circuit gets smaller, the driving frequency for the ultrasonic head cannot reach the target resonant frequency in a shorter period. The driving signal for the ultrasonic head accordingly cannot be raised.